bridge: Reset IPCB when entering IP stack on NF_FORWARD

bridge: Reset IPCB when entering IP stack on NF_FORWARD

Whenever we enter the IP stack proper from bridge netfilter we
need to ensure that the skb is in a form the IP stack expects
it to be in.

The entry point on NF_FORWARD did not meet the requirements of
the IP stack, therefore leading to potential crashes/panics.

This patch fixes the problem.

Signed-off-by: Herbert Xu <[email protected]>
Acked-by: Stephen Hemminger <[email protected]>
Signed-off-by: David S. Miller <[email protected]>